1. Field of the Invention
The present invention relates to a printing plate material and more particularly to one which provides a printing plate having an improved printing durability.
2. Description of the Prior Art
In general, a lithographic printing plate consists of the oleophilic ink-adhering portions for forming an image area and the hydrophilic portions for forming a non-image area. In the manufacture of such a printing plate, there is known a process in which only the image area of the hydrophilic surface is converted to become oleophilic and a process in which a hydrophilic layer is formed on the surface of the oleophilic substance and then only the hydrophilic layer of image area is removed to expose the oleophilic surface. As an example of a process belonging to the latter, a process is described in British Pat. No. 1,129,366, wherein a silver image is formed on the hydrophilic layer of the sensitive plate by diffusion transfer development, and the sensitive plate a hydrophilic layer containing the nucleus substance for the diffusion transfer process is formed on the support and has an oleophilic surface, a silver halide light-sensitive emulsion is coated thereon, and the hydrophilic layer for the corresponding area is removed through the etching-bleach treatment using the silver image to expose the oleophilic support resulting in a printing plate. Another process is described in U.S. Pat. No. 3,385,701, wherein an exposed negative material is contacted to a sheet and subjected to diffusion transfer development to form a silver image on the hydrophilic layer, the sheet containing a hydrophilic layer containing nucleus substance for the diffusion transfer process on a support having an oleophilic surface, and the hydrophilic layer for the corresponding area is removed through the etching-bleach treatment using the silver image to expose the oleophilic support resulting in a printing plate.
However, when these known printing plates are used in practice for printing, there is found a disadvantage in that the background becomes spotty due to a separation of the hydrophilic layer which is apt to take place and a good quantity of printed matter of good quality is difficult to obtain. For example, as to the printing plate disclosed in British Pat. No. 1,129,366, when the hydrophilic image-receiving layer is coated directly on the surface of oleophilic support, if the adhesion between the support surface and the hydrophilic layer is not sufficient, the hydrophilic layer of non-image area is apt to be separated on printing and the adhesion thereto of oily ink results in background spots.
For the common lithographic printing plate in which a hydrophilic layer is formed directly on the oleophilic surface, the hydrophilic layer corresponding to the image area is removed to expose the oleophilic surface, and there is a difference between the affinity to an oily ink of said oleophilic surface and the hydrophilic layer, in order to prevent the occurrence of background spots resulted from the adhesion of ink to the non-image area by making the difference in the affinity to ink as great as possible. This can be attained by making the hydrophilic property of said hydrophilic layer as high as possible. However, as a result, the adhesion between the oleophilic surface and the hydrophilic layer grows worse and the non-image area of the hydrophilic layer is itself separated to yield the background spots. It has been long desired by those skilled in the art to provide a printing plate free from the formation of background spots and also having a good printing durability by overcoming these problems.